1. Field of the Invention
The present invention relates to a catalyst system to be used in an automobile exhaust gas purification apparatus, an exhaust gas purification apparatus using the same, and an exhaust gas purification method, and in more detail, the present invention relates to a catalyst system to be used in an automobile exhaust gas purification apparatus which exerts excellent purification capability to a nitrogen oxide, even when hydrocarbon concentration varies, by subjecting exhaust gas discharged from an automotive internal engine to contacting with a catalyst, an exhaust gas purification apparatus using the same, and an exhaust gas purification method.
2. Description of the Prior Art
Exhaust gas discharged from an internal combustion engine of an automobile or the like, or a combustion engine of a boiler or the like, contains toxic substances such as a hydrocarbon (HC), carbon monoxide (CO), a nitrogen oxide (NOx), and various exhaust gas purification technologies for purifying these have been proposed. As one of these, there has been studied exhaust gas purification technology for purifying toxic components in exhaust gas, by installment of a catalyst in exhaust gas passage.
In particular, in the case where exhaust gas from an automobile is purified by such catalyst technology, there has been required performance which is capable of flexibly corresponding to abrupt variation of concentration of these toxic gases (CO, HC, NOx) ranging from several ppm to several %, and is capable of purifying exhaust gas in high efficiency.
In the exhaust gas purification technology used such a catalyst, oxygen present in exhaust gas has an important role of oxidizing CO and HC, as well as CO, HC etc. present in exhaust gas has an important role of promoting a reductive reaction of NOx. However, in the case of an automobile, because of variation of running circumstances depending on a road jam state, it is difficult to maintain an engine combustion state constant, and oxygen concentration in exhaust gas also varies every second. Accordingly, there was a case where purification of the toxic substances by utilization of oxygen became impossible when oxygen concentration was lowered.
In order to overcome this problem, it is carried out to enhance purification capability of toxic substances, by blending an oxygen storage component (it may be referred to as the OSC material) into a composition of the exhaust gas purification catalyst, so as to supply oxygen into exhaust gas, when oxygen concentration is low.
CeO2 powder has been used as the OSC material of this catalyst, because of having high storage and discharging capability of oxygen, and it has been clarified to increase exhaust gas treatment efficiency. Up to now, many investigations have been made on oxygen storage capacity, enhancement of discharging characteristics in CeO2-type powder, such as a CeO2—ZrO2-type or the like, and on an exhaust gas purification catalyst using this as a co-catalyst.
As such an exhaust gas purification catalyst, it has been approved that a cerium-zirconium composite oxide with specific atomic ratio is effective as an oxygen storage component for the exhaust gas purification catalyst, to adjust oxidizing action of HC and Co in exhaust gas, or reducing action of NOx.
In addition, in the case of an exhaust gas purification catalyst to be mounted on an automobile, there are two types, that is, one installed under a floor where temperature is relatively low, and what is called a just-under-type, installed at a position exposed to high temperature exhaust gas, just after discharged from an engine. However, even exhaust gas, whose temperature was high just after discharged from an engine, temperature thereof becomes further lower till reaching under the floor. Because an exhaust gas purification catalyst generally exerts high activity at higher temperature than a certain level, in many cases, this state is not a preferable condition to purification of exhaust gas. However, a catalyst to be used as the under-the-floor type is required to exert purification capability, even at such a low temperature state.
On the other hand, the just-under-type may encounter a case where temperature of exhaust gas exceeds 1000° C., which may incur sintering of exhaust gas purification catalyst components. Therefore, the exhaust gas purification catalyst is required to have performance for suppressing sintering of the catalyst components and purifying stably the exhaust gas, even under strict condition at such high temperature.
As the automobile exhaust gas purification catalyst, a catalyst for carrying out oxidation of CO and HC, and reduction of NOx, at the same time by a single catalyst, has been known as a three way catalyst (TWC) (hereafter the automobile exhaust gas purification apparatus using the TWC may be referred to as the TWC apparatus). A constitution of this TWC is generally, for example, one where a structural type carrier composed of cordierite is coated with slurry in which noble metal elements such as platinum, rhodium and palladium is supported on a base material of a porous carrier composed of alumina.
In such a TWC, it is generally carried out oxidative removal of reducing components such as CO, HC by a reaction with oxygen in exhaust gas, or reductive purification of NOx by utilization of CO, HC etc. in exhaust gas, however, as described above, because oxygen concentration in exhaust gas discharged from an automobile varies every second, a catalyst exerting purification capability, even under such environment, has been required. Under such circumstances, it has been carried out to enhance purification capability of toxic components, by formulating a cerium oxide or a cerium-zirconium-type composite oxide (Patent Literature 1) into a composition of an exhaust gas purification catalyst as oxygen storage and discharge material, and supplying an oxygen into exhaust gas when oxygen concentration decreases, mainly aiming at buffering of such variation of oxygen concentration in exhaust gas.
In addition, there have been proposed an exhaust gas purification catalyst (Patent Literature 2) having excellent ignition performance at low temperature that expresses catalytic action at low temperature, which is a catalyst including catalytically activated components containing the noble metal components, the refractory inorganic oxides, and the zirconium oxides containing cerium and additives, wherein crystal of the zirconium oxide containing cerium and the additives has a single structure of a tetragonal structural type zirconium oxide; still more an exhaust gas purification catalyst (Patent Literature 3) having both large specific surface area and high oxygen storage and discharge capability, which is a composite oxide of CeO2 and ZrO2, having any one or more kind of a pyrochlore phase, a K-phase or an intermediate phase thereof.
However, with tightening of exhaust gas regulations in recent years, the market has required the OSC material which is capable of discharging oxygen at low temperature region, and has higher oxygen storage and discharging capability and long life catalyst performance, therefore, an exhaust gas purification catalyst with excellent resistance to high temperature has been desired. Under these circumstances, the present inventors have developed the OSC material having excellent performance even at high temperature, and have proposed a cerium-zirconium composite oxide not being sintered, even at a high temperature of 1000° C. or higher (refer to Patent Literature 4).
In addition, in the automobile exhaust gas purification apparatus, there may be the case where a catalyst having function corresponding to exhaust gas components of a purification subject is arranged at a plurality of places of exhaust gas passages, and constitutes the exhaust gas purification apparatus.
Also in a gasoline engine, it is often carried out to arrange two or more TWCs themselves in an exhaust gas passage. For example, there have been investigations to completely oxidize partially oxidized HC, by arrangement of a high heat resistant catalyst at the former stage which is exposed to high temperature exhaust gas, while by arranging the usual TWC at the later stage (Patent Literature 5); and to enhance largely HC purification rate in exhaust gas, by arrangement of the TWC at the former stage, and by arrangement of a composite catalyst compounded of an HC absorbing agent and the TWC at the later stage (Patent Literature 6).
In addition, in the case where a catalyst is arranged in an exhaust gas passage with limited space and shape, in order to obtain sufficient activated surface for exhaust gas purification, there may be the case where the same kind of TWCs are arranged dividedly at two or more places. Patent Literature 6 is the case of exhaust gas purification by the TWC apparatus constituted by using two or more catalysts.
In addition, in the conventional TWC, it has been considered that theoretical air/fuel ratio, called a window, is required to be an extremely narrow range of near 14.6, to make possible suitable purification of three components of HC, CO and NOx. However, in an automobile, due to increased concern to an environmental problem in recent years, in addition to the above running condition, it is inevitable to operate in a lean state such as fuel cut (to stop temporarily fuel supply into a combustion chamber), lean combustion (it may be referred to also as lean burn), in order to enhance fuel economy. Such a state of large theoretical air/fuel ratio results in increase in NOx generation amount, and purification thereof was difficult by the conventional TWC.
In addition, also from a view point of an oil resource depletion issue and a global warming issue, attainment of low automotive fuel cost has been expected, and concern to a lean combustion engine has been increased also in a gasoline automobile. In the lean combustion, exhaust gas atmosphere in running becomes oxygen excess atmosphere (hereafter may be referred to as “lean atmosphere”), as compared with theoretical air/fuel ratio (hereafter may be referred to as a “stoichiometric state”). In the case where the conventional TWC is used in the lean atmosphere, there was a problem that a large quantity of NOx is discharged, due to influence of excess oxygen, resulting in insufficient purification action. Therefore, there has been investigated on a catalyst which is capable of purifying NOx even under the lean atmosphere (Patent Literature 7).
However, there has been ever increasing strict request for enhancement of automotive fuel economy in recent years, and frequency of operation under the lean atmosphere or fuel cut has been increasing, however, degree of severity in request to NOx purification has been increasing also, with increase in environmental consciousness, and in particular, still more improvement of NOx purification performance of the TWC has been desired.    [Patent Literature 1] JP-B-6-75675    [Patent Literature 2] JP-A-2002-336703    [Patent Literature 3] JP-A-2005-170774    [Patent Literature 4] WO2006/030763    [Patent Literature 5] JP-A-11-123331    [Patent Literature 6] JP-A-7-144119    [Patent Literature 7] JP-A-10-192713, paragraph [0002]